The present invention relates to an improved type of liquid crystal display panel having an individual switching element coupled to each display element of the display panel.
Liquid crystal display panels have been developed in order to provide a display of relatively large amounts of data, in devices in which power consumption must be held to the lowest possible level. Such devices include electronic timepieces, electronic calculators, ultra-small television receivers, etc. These liquid crystal display panels are basically of matrix configuration, with row and column electrodes being arranged on one substrate of the display panel and a display electrode being provided at each intersection of the row and column electrodes. A planar electrode is usually provided on the opposite substrate of the display panel, with the liquid crystal material being enclosed between the two substrates. The planar electrode is held at some fixed, reference potential, so that when a display signal is applied to a particular display electrode by means of the row and column electrodes, an electric field is produced between the display electrode and the reference potential electrode, whereby the portion of liquid crystal to which this electric field is applied is energized, in other words the corresponding display element is made visible. In order to provide a display panel which has a large number of display elements, it is necessary to perform time-division multiplexing of the signals applied to the row and column electrodes of the display panel. However, due to the characteristics of the liquid crystal material, it has been found that in practice it is necessary to adopt a system in which a single switching element is coupled to each display element of the display panel, in order to permit reasonably high-level multiplexing of the signals applied to the row and column electrodes. This switching element enables a display signal to be applied to a selected display electrode, and to thereafter isolate that display electrode so that the charge resulting from the display signal voltage is left stored on the display electrode. Such a type of liquid crystal display panel, in which switching elements (e.g. field-effect transistors) are "built into" the display panel has been described in the prior art by B. J. Leichner et al (Proc. IEEE, Vol. 59 , Nov. 1971, p. 1566 to 1579). This type of display panel offers the important advantage that, even if the level of multiplexing of the signals applied to the row and column electrodes is comparatively high, there is essentially an absence of cross-talk effects. However, prior art examples of such "built-in switching element" type display panels have various disadvantages. These are associated with the fact that the row and column electrodes are formed upon one substrate of the display panel, and a planar reference potential electrode (referred to hereinafter for convenience as a reference electrode) is formed upon the opposing substrate. Thus, substantial levels of stray capacitance exist between the row and column electrodes and the reference electrode. As a result, if the speed of switching of the signals applied to the row and column electrodes is comparatively high (as will be necessitated by high-level multiplexing), then the amount of power which is consumed as a result of charging and discharging the stray capacitances will also be high. Thus, with a conventional type of liquid crystal display panel of the "built-in switching element" type, it is difficult to provide a large number of display elements, or to reduce the number of connecting leads to the various electrodes, by increasing the speed of switching of display signals. However, it is highly desirable to be possible to increase the speed of switching of the display signals of such a liquid crystal display panel, so that the number of interconnecting leads between the display signal circuitry and the display panel electrodes can be reduced, in order to facilitate the manufacture of such display panel systems in an economic manner.
These problems are substantially alleviated by a liquid crystal display panel constructed in accordance with the present invention. As stated above, the disadvantages of such prior art types of liquid crystal display panels are basically due to the fact that row and column electrodes, carrying signals which are switched at a comparatively high frequency, are formed on one substrate of the display panel, while the reference electrode is formed as a planar electrode upon the opposing substrate, so that an electric field is generated in a direction which is perpendicular to the substrate plane, in order to activate a particular display element. With the present invention, however, both the row and column electrodes, and also the reference electrode, are formed upon a single substrate. In order to activate a particular display element, an electric field is applied between a reference electrode and a display electrode, each of which is arranged in the form of a comb-shaped pattern, with the electrode patterns being mutually intermeshed as will be made clear by the description hereinafter of specific embodiments of the present invention. As a result, the degree of stray capacitances which arise between the various electrodes of the display panel is substantially reduced, by comparison with liquid crystal display panels of the prior art, so that the frequency of switching of signals applied to the display panel electrodes can be signficantly increased without an excessive increase in power consumption. A liquid crystal display panel according to the present invention can therefore utilize a high degree of multiplexing of the signals applied to the row and column electrodes, whereby the number of interconnecting leads between the source of drive signals for the row and column electrodes and these electrodes can be substantially reduced. Manufacture of a display system including a display panel according to the present invention therefore is made easier and more economical, by comparison with liquid crystal display panels of the prior art.